It is known in the literature and in the industry that there are at least four different feasible methods for the production of acrylic-based pressure-sensitive adhesive (hereinafter "psa") tapes. These known methods include solution polymerization, emulsion polymerization, irradiation by high energy particulate matter (e.g., electron beams or gamma rays), and ultraviolet light (hereinafter "UV") photopolymerization. As explained below, however, there are disadvantages and/or limitations incurred with the use of each known process.
When utilizing psa's made by solution polymerization, elaborate drying ovens with massive exhaust ducts and high temperatures are required to carry away the volatile solvents after coating. Furthermore, to prevent the solvents from being vented to the atmosphere (with resulting pollution and solvent loss), expensive solvent recovery equipment has been necessary. Safety hazards in such operations are also severe, as the solvents are extremely flammable and precautions must be taken to avoid explosive mixtures in the oven and exhaust systems. A further limitation of the solvent based systems is the limit on the thickness of the coatings which can be deposited in one trip or pass through the coater. Thus, with coatings above about 5 mils, multiple coating layers must be deposited in successive trips through the coater to avoid blistering of the coating due to solvent evaporation.
While emulsion polymerization has eliminated the problems associated with the handling and evaporation of flammable solvents, the heat of vaporization must be supplied to remove the water from the coating and essentially the same equipment must be employed. Though higher solids coatings are possible, the higher heat of vaporization of water as compared to organic solvents offsets this benefit and about the same total energy for drying is required. Drying times are relatively long, thus limiting production. One of the most serious limitations of the emulsion polymerization process is the water sensitivity of the resulting polymers (caused by the emulsifying agent which is carried along in the process and becomes part of the final polymer). A further limitation of this process is that highly polar monomers, which are water miscible, are difficult to incorporate into the copolymer during polymerization and considerable homopolymerization of such monomers can occur in the aqueous phase.
Various attempts have been made to avoid the difficulties of the solution and emulsion polymerization processes. However, those have thus far resulted either in processing difficulties of their own or have produced polymers in which a proper balance between compliance and cohesive strength is very difficult to control.
More recently, development work has been done with polymerization processes which employ either ultraviolet light or electron beams. One patent which stresses electron beam curing is U.S. Pat. No. 3,897,295, in which the composition subject to the electron beam includes an acrylate monomer selected from a particular specified group, and a homopolymer or copolymer of a substance or substances selected from the same group. The polymer is dissolved in the monomer and the monomer is ultimately polymerized to bind the adhesive together.
The disadvantage of utilizing polymerization processes involving an electron beam, though, is that, generally, it is a rather indiscriminate polymerization process. In polymerization processes utilizing an electron beam, the particulate bombardment of the free-radically polymerizable monomers cannot be precisely controlled, with the result being chain scission of the developing polymer and an inability to control its molecular weight and crosslink density to the most desired range.
In order to avoid the above-discussed disadvantages incurred with the use of an electron beam, some have chosen to utilize a one step or stage low-intensity (e.g., 0.1 to 7 mW/cm.sup.2) UV photopolymerization process. See, for example, U.S. Pat. No. 4,181,752. Whereas the use of relatively low intensity UV light is very desirable for building higher molecular weight acrylic psa's with good performance properties, an increase in the speed of the photopolymerization process would be desirable. However, if one attempts to increase the speed of the low intensity UV light-based process by increasing the amount of the photoinitiator employed (e.g., benzoin ethers, benzil ketals, etc.), then undesirable lower molecular weight polymers will be obtained. Furthermore, for thick adhesives an uneven polymerization from the front surface to the back surface of an irradiated adhesive composition occurs due to the uneven light absorption by the photopolymerization initiator resulting in a differential performance of the final psa product.
In view of the foregoing discussed disadvantages and limitations that exist with the use of conventional polymerization processes, improvements are continuously desired and sought by those within the industry. It was against this background that an improved polymerization or irradiation process for producing acrylic-based adhesives, and in particular acrylic-based psa tapes was sought.